Tracked as APT41, the threat actor has been active since at least 2012, and has been engaging in both state-sponsored espionage activities and financially-motivated operations since 2014. It has been observed targeting the gaming, healthcare, high-tech, higher education, telecommunications, and travel services industries.

Unlike other Chinese threat groups, APT41 was seen using non-public malware typically reserved for espionage campaigns in cyber-crime attacks. The hackers move laterally within the compromised networks, pivoting between both Windows and Linux systems, and are known to have used over 46 different malware families and tools in their operations.

In April 2019, the group targeted a publicly-accessible web server at a U.S.-based research university, exploiting CVE-2019-3396, a vulnerability in Atlassian Confluence Server, for path traversal and remote code execution.

The attackers used custom JSON POST requests to run commands and force the vulnerable system to download an additional file, which was identified as a variant of the China Chopper web shell.

Next, the attackers downloaded two additional files onto the system, the first of which was used to deploy the HIGHNOON backdoor, which consists of a loader, a dynamic-link library (DLL), and a rootkit. The DLL may deploy additional drivers to conceal network traffic and to fetch memory-resident DLL plugins from the command and control (C&C) server.

Within the next 35 minutes, the attackers used both the China Chopper web shell and the HIGHNOON backdoor to issue commands to the system.

The HIGHNOON backdoor was used to execute a PowerShell command and download a script from PowerSploit, a well-known PowerShell post-exploitation framework. The script was likely a copy of Invoke-Mimikatz, reflectively loading Mimikatz 2.0 into memory.

The attackers were also observed performing additional reconnaissance and downloading two additional files, representing the dropper and encrypted/compressed payload components of the ACEHASH malware, a credential theft and password dumping utility.

“Ultimately, the attacker was able to exploit a vulnerability, execute code, and download custom malware on the vulnerable Confluence system. While Mimikatz failed, via ACEHASH they were able to harvest a single credential from the system. However, as Managed Defense detected this activity rapidly via network signatures, this operation was neutralized before the attackers progressed any further,” FireEye notes.